Add BT Northern Ireland outcodes

[iamhosseindhv]

As mentioned in #2 , BT outcodes are missing. In case it's of any interest, this PR adds a BT.geojson file derived from BT.kml file attached below. Source of these files are https://www.doogal.co.uk/UKPostcodes.php?Search=BT

BT.kml.zip

[missinglink]

Thanks @iamhosseindhv this is great.

Before we merge this I'd like to add:

• A script which could be used in the future to update the data.
• Clarity on the licence conditions of the data (for anyone who consumes the data).

[missinglink]

Hi @iamhosseindhv are you still up for merging this?
We can do without the script but would need some clarification on the liscensing